CONSTANT BUFFERING

HELP! For the past 4 months or so, I haven't been able to watch any video on my computer with Comcast without it buffering every 5 seconds. It then lasts for approximately 20 seconds. I know because I timed it watching The Walking Dead. 5 seconds (equal to about 3 words) then waited 20 seonds for the partial completion of the sentence. This DOES NOT happen when I watch stuff on Netflix, so what does this mean? I have defragged, emptied cookies, gotten Webroot, etc. HELP!